This invention pertains to telecommunications, and particularly to wireless communications relating to whether a wireless terminal should operate in a single antenna or multiple antenna (e.g., Multiple-Input Multiple-Output (MIMO)) mode.
In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability (e.g., mobile termination), and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB” or “B node”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN, short for UMTS Terrestrial Radio Access Network, is a collective term for the Node B's and Radio Network Controllers which make up the UMTS radio access network. Thus, UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. One result of the forum's work is the High Speed Downlink Packet Access (HSDPA) for the downlink, which was introduced in 3GPP WCDMA specification Release 5. The High Speed Downlink Packet Access (HSDPA) was followed by introduction of High Speed Uplink Packet Access (HSUPA) with its Enhanced Dedicated Channel (E-DCH) in the uplink in 3GPP WCDMA specification Release 6.
High Speed Downlink Packet Access (HSDPA) achieves higher data speeds by, e.g., shifting some of the radio resource coordination and management responsibilities to the base station (RBS) from the radio network controller (RNC). Those responsibilities include one or more of the following: shared channel transmission, higher order modulation, link adaptation, radio channel dependent scheduling, and hybrid-ARQ with soft combining.
In accordance with the first of the shifted responsibilities, i.e., shared channel transmission, HSDPA multiplexes user information for transmission on the high-speed downlink shared channel (HS-DSCH) in time-multiplexed intervals (called transmission time intervals (TTI)) over the air interface to the mobile terminal. Three new physical channels were introduced with HSDPA to enable HS-DSCH transmission. Of these three, the high-speed shared control channel (HS-SCCH) is a downlink control channel that informs mobile devices when HSDPA data is scheduled for them, and how they can receive and decode it (e.g., the HS-SCCH provides timing and coding information allowing the wireless terminal to listen to the HS-DSCH at the correct time and using the correct codes). The high-speed dedicated physical control channel (HS-DPCCH) is an uplink control channel used by the mobile to report the downlink channel quality and request retransmissions. The high-speed physical downlink shared channel (HS-PDSCH) is a downlink physical channel that carries the HS-DSCH user data. Several HS-PDSCHs are assigned to a mobile for each transmission. Each HS-PDSCH has a different orthogonal variable spreading factor (OVSF) channelization code.
Multiple-Input Multiple-Output (MIMO) transmission schemes can be utilized to increase spectral efficiency. MIMO schemes assume that the transmitter and receiver are both equipped with multiple antennas, and that multiple modulated and precoded signals can be transmitted on the same “time-code resource”.
Evolved HSPA (also known as: HSPA Evolution) is a wireless broadband standard defined in 3GPP release 7. HSPA+ provides HSPA data rates up to 28 Mbit/s on the downlink and 11 Mbit/s on the uplink with MIMO technologies and higher order modulation. HSPA+ supports 2×2 downlink MIMO that uses two transmit antennas at the Node B to transmit orthogonal (parallel) data streams to the two receive antennas at the wireless terminals. Using two antennas and additional signal processing at the receiver and the transmitter, MIMO can increase the system capacity and double user data rates without using additional Node B power or bandwidth.
The foregoing highlights the fact that some wireless terminals can operate either in a MIMO mode (which involves, e.g., two or more streams/channels of transmission from the base station) or a non-MIMO mode (one stream/channel of transmission from the base station).
One drawback with operation in the MIMO mode in high speed downlink packet access (HSDPA) is the extra overhead introduced in the related control signaling. For example, the MIMO downlink (DL) control channel, i.e., the high speed-shared control channel (HS-SCCH), may require two decibels (2 dB) more transmit power than the non-MIMO HS-SCCH. The extra two decibel or so power requirement occurs because more information needs to be transmitted to a user in MIMO mode, e.g., precoder weights and modulation information for up to two transport blocks must be signaled. Also in the MIMO mode the uplink (UL) overhead increases for MIMO users because the channel quality indicator (CQI) and hybrid Automatic Repeat-reQuest (HARQ) ACK/NACK information for two streams must be fed back to the base station.
For reasons such as those mentioned above, it is desirable to have wireless terminals operate in the MIMO mode only when the probability or need for dual stream transmission (e.g., use of multiple antennas by the wireless terminal) is sufficiently high. For example, if a wireless terminal reports that only one stream can be supported for some time, it is desirable to turn off the MIMO mode transmission for the reporting wireless terminal since the overhead needed for MIMO transmission does not pay off in increased performance. On the other hand, for a MIMO-capable wireless terminal which is operating in a non-MIMO mode, there currently is no good way to switch that wireless terminal into MIMO mode if conditions favor MIMO transmission.
The radio network controller (RNC) makes the determination for switching a wireless terminal between the MIMO mode and the non-MIMO mode. The mode in which the wireless terminal is to operate is sent over higher layers to the wireless terminal, and consequently the mode switching changes very slowly. Unfortunately, in conventional practice the radio network controller (RNC) does not know what quality each wireless terminal is experiencing.
A wireless terminal makes various measurements and reports to a base station certain channel quality indicator (CQI) reports. The channel quality indicator (CQI) reports received at a base station from a wireless terminal can be used by the base station scheduler to adapt the current transmissions to the wireless terminal to the actual channel conditions. In the current version of the WCDMA specification, Release 7, incorporated herein by reference, several different versions of CQIs exists to support different operation modes. For example, a “normal” CQI is used for wireless terminals not configured in multiple-input-multiple-output—(MIMO) mode, while either type A or type B CQI reports are used for MIMO-capable UE terminals operating in MIMO mode, i.e., capable of receiving multiple transmission streams using multiple antennas.
The normal CQI consists of five bits and serves as a representation of the received signal-to-noise ratio (SNR) that corresponds to a 10% block error rate (BLER) at the wireless terminal. For MIMO-capable wireless terminals, type A and B CQIs also include preferred precoder weights in addition to quantized signal to noise ratio (SNR) values. When a MIMO-capable wireless terminal is not configured in MIMO mode, the normal CQI report is used.
As mentioned above, currently there is no good way for a MIMO-capable wireless terminal currently in a non-MIMO mode to switch into MIMO mode if conditions favor MIMO transmission. One approach might be to monitor the non-MIMO CQI of that wireless terminal and then turn-on MIMO mode at the wireless terminal when a sufficiently high SNR is reported. However, a problem with this approach is that a normal CQI does not contain any antenna-related information. As a result, even if the SNR reported in the CQI were high, the two channels could nevertheless be sufficiently correlated that they would not be appropriate for MIMO transmission.